1. Field of the Invention
The present invention relates to an optical communication system, and more particularly, to an optical wavelength locking apparatus and method for a multi-channel optical communication system to stabilize the wavelength of an optical source.
2. Background of the Related Art
In general, in order to obtain ultra-high speed communication through an optical fiber, an optical communication system employs an optical multiplexing method such as optical time division multiplexing (OTDM), wavelength division multiplexing (WDM), or optical frequency division multiplexing (OFDM).
Among the multiplexing methods, the WDM method is used to divide/transfer an input channel to mutually different wavelength bands (e.g., 1.3 mm and 1.5 mm). WDM has advanced to a dense WDM (DWDM) method in which the wavelength bands are more closely spaced. DWDM can be used, for example, to divide/transfer the 1.5 mm wavelength band to a plurality of closely spaced wavelength bands.
Typically, an optical communication system employing the DWDM method uses an optical wavelength locking apparatus to restrain losses between optical channels. In this respect, if the wavelength spacing between optical channels is 0.4 nm, an optical wavelength locking circuit is needed in order to restrain wavelength fluctuations of the transmission optical source to within ±20 pm.
FIG. 1 is a block diagram of an optical wavelength locking apparatus for a single-channel system, in accordance with the related art.
As shown in FIG. 1, a related art optical wavelength locking apparatus for a single-channel system includes a laser diode 10 for converting transmission data into an optical signal, an optical tap 12 for separating a portion of the optical signal output from the laser diode 10, an optical wavelength locking unit 14 for analyzing the separated optical signal and outputting a control signal (ITEC) to control a wavelength fluctuation (δλ) of the optical signal, and a thermoelectric cooler (TEC) 16 for controlling the temperature of the laser diode 10 according to the control signal (ITEC) output from the optical wavelength locking unit 14.
The optical wavelength locking unit 14 includes an optical wavelength filter (OWF) 50 for outputting current signals (IA, IB) in proportion to the wavelength fluctuation (δλ1) of the optical signal, and a control circuit 52 for outputting a control signal (ITEC) to drive the TEC 16 according to the current signals (IA, IB) output from the OWF 50.
The operation of the conventional optical wavelength locking apparatus for a single-channel system constructed as described will now be explained.
The laser diode 10 converts data to be transmitted into an optical signal, and outputs the optical signal to an optical fiber 11. The optical tap 12 separates a portion of the optical signal [P(λ+δλ)] transmitted through the optical fiber 11 and sends it to the optical wavelength locking unit 14.
The OWF 50 of the optical wavelength locking unit 14 receives the optical signal [P(λ+δλ)] output from the optical tap 12 and outputs current signals (IA, IB) in proportion to the wavelength fluctuation (δλ). The control circuit 52 generates a control signal (ITEC) according to a difference between the current signals (IA, IB). The difference between the current signals (IA, IB) is proportional to the wavelength fluctuation (δλ).
Accordingly, the TEC 16 controls a temperature of the laser diode 10 according to a control signal (ITEC) output from the control circuit 52. That is, the TEC 16 is cooled or heated according to a polarity of the control signal (ITEC), and thereby controls the temperature of the laser diode 10. By adjusting the temperature of the laser diode 10, the wavelength fluctuation (δλ) of the optical signal emitted from the laser diode 10 may be controlled.
As described above, the related art optical wavelength locking apparatus for a single-channel system includes one optical wavelength locking unit 14 for one transmission optical source (e.g., laser diode 10) to control the wavelength fluctuation of the optical signal emitted from the laser diode 10.
FIG. 2 is a block diagram illustrating a related art optical wavelength locking apparatus for a multi-channel system. As shown in FIG. 2, the related art optical wavelength locking circuit for a multi-channel system includes a plurality of optical wavelength locking units, with each optical wavelength locking unit being similar to the one used in the single channel system illustrated in FIG. 1.
That is, optical taps (12-1˜12-N), optical wavelength locking units (14-1˜14-N) and TECs (16-1˜16-N) are respectively used for each of a plurality of laser diodes (10-1˜10-N), so as to control the wavelength fluctuation of optical signals emitted from each of the plurality of laser diodes (10-1˜10-N). Thus, in the related art optical wavelength locking apparatus for a multi-channel system, an optical wavelength locking unit is provided for every laser diode. Accordingly, if there are N number of transmission channels, then N number of optical wavelength locking units are required.
The optical wavelength filter used in each optical wavelength locking unit is a very expensive precision optical component. Thus the production costs involved in implementing an optical wavelength locking apparatus in a multi-channel system are high.
In addition, in the related art optical wavelength locking apparatus for a multi-channel system, since every laser diode needs a respective optical wavelength locking unit, installation space is increased. Further, since the wavelength fluctuation of optical signals in each channel is controlled by a different optical wavelength locking unit, the reliability of the system is degraded.
Another related art optical wavelength locking apparatus for a multi-channel system uses an arrayed waveguide grating (AWG). However, AWGs have limited wavelength resolution and precision.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.